Scientists at the University of Queensland in Brisbane took cores from three reef sites and determined corals collapsed between the 1920s and 1950s, NewScientist.com reported.
Two of the sites had little coral left after the 1950s, while the third had been colonized since then by different types, researchers said.
By the 1920s, European settlers were farming intensively near rivers flowing onto the reef, boosting agricultural run-off that killed coral, the researchers said.
And the extra nutrients in the water help seaweed move in afterwards, preventing coral from regenerating, Terry Done of the Australian Institute of Marine Science in Townsville, Queensland, said.
The researchers say their work showed the reefs had been in pristine condition prior to the 1920s.